Caption: Tamoxifen cancer drug molecule. This drug is used to treat breast cancer. It is a selective oestrogen receptor modulator, a class of drug that works by binding to the oestrogen receptor on cancer cells, where it inhibits cell growth. It is also taken as a drug of abuse by male bodybuilders, as it prevents the gynaecomastia (male breast growth) caused by steroid drugs taken to build muscle mass. The atoms are shown as spheres and are colour-coded: carbon (taupe), hydrogen (white), oxygen (plum) and nitrogen (emerald).